CALCRUST NORTH: Tectonics of the California Coast Ranges in the San Francisco Bay Area

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This research program is an investigation of the present crustal tectonics and the evolution of the central California coast ranges in the San Francisco Bay region, where the San Andreas fault system splays eastward through a complex of successive bifurcations into the Calaveras, Hayward and Greenville faults, a process readily visible in the seismicity maps of the area. Competing hypotheses for crustal accommodation of the plate boundary motion will be tested. Evidence will be sought on the compelling modern paradigm for the crustal dynamics of this system, in which upper crustal blocks along with their bounding faults move eastward, above a mid-crustal decollement, creating the regional complex of east- and west-vergent thrust faults, tectonic wedges and fold belts. The proposed research is an integrated investigation of the crust's third dimension eastward from San Francisco Bay to the western boundary of the Great Valley. Modern seismological methods and geological/geodetic constraints will be combined in the study. Reflection seismic profiles, natural-event P- and S-wave tomography, ongoing geological studies of East Bay faults, and new data on crustal deformation in the Bay Area will be interpreted jointly within a common geometrical framework for the plate boundary in central California. Reflection data are key to this study, and a primary focus in this project. 160 km of high- quality industry "spec" data in the area are available for purchase and can be reprocessed for deeper imaging. The new BASIX (Bay Area Seismic Imaging eXperiment) marine/land profile, shot in the Bay, River and offshore waters with USGS support in September 1991, provides additional valuable coverage of the region, and the data are in hand at Berkeley.